stiff·en / ˈstifən/ • v. make or become stiff or rigid: [tr.] he stiffened his knees in an effort to prevent them from trembling | [intr.] my back stiffens up and I can't bend. ∎ [tr.] support or strengthen (a garment or fabric), typically by adding tape or an adhesive layer. ∎ fig. make or become stronger or more steadfast: [tr.] outrage over the murders stiffened the government's resolve to confront the Mafia | [intr.] the regime's resistance stiffened. DERIVATIVES: stiff·en·er / ˈstif(ə)nər/ n.